1. Field of the Invention
This invention relates to rotor blade shrouds and more specifically to reducing centrifugal stresses experienced by rotor blade components during operation.
2. Description of Related Art
Rotor assemblies are used in a variety of turbo-machines, such as turbines, compressors and the like. Regardless of the application, as shown in FIG. 1, typical rotor assembly 2 construction includes a disk 4 mounted on a rotating shaft 6 with a plurality of blades 8 extending radially outward around the circumference of the disk. The blades are normally mounted on the disk in a dovetail slot 9 designed to match the dovetail root portion 10 of the blade. Extending radially from the root portion, the blade also includes a platform portion 12, an airfoil portion 14 and a shroud 20, as shown in FIG. 2. The airfoil portion includes a generally concave high-pressure sidewall 22 and a generally convex low-pressure sidewall 24 that extend axially from the blade leading edge 26 to the blade trailing edge 28. The airfoil sidewalls extend radially between the platform portion of the rotor blade and the shroud.
Flow of the working fluid, i.e., air for jet engines, across the airfoil portion imparts a force upon the blades and results in rotation of the shaft, thereby converting the thermal and kinetic energy of the working fluid into mechanical rotation of the rotor. Rotor efficiency is improved by minimizing leakage of the working fluid through the gap between the blade and the stationary housing (not shown) surrounding the rotor. One commonly used design to prevent this leakage is the use of a shroud. The shroud is located at the radially distal end of the rotor blade adjacent the stationary housing and is designed to minimize the flow of working fluid through the gap between the rotor blade and the stationary housing. Shrouds are also designed to minimize the flow of working fluid in the space between circumferentially adjacent shrouds.
FIG. 3 is a top view showing conventional shrouds 20 including generally parallel leading 30 and trailing edges 32, oriented generally perpendicular to the working fluid flow path, and first 34 and second 36 circumferential sides. The shroud circumferential sides are generally contoured to complement the circumferentially adjacent shrouds, thereby preventing the flow of working fluid between adjacent shrouds. Conventional shrouds also typically include one or more narrow sealing rails or knife edges 38 that extend radially outward from the shroud in close proximity to the stationary housing and typically extend continuously across the top surface of the shroud between first and second circumferential sides. The sealing rail minimizes leakage of the working fluid through the gap between the rotor assembly and the stationary housing. Sealing rails further provide sacrificial material in the event of contact with the stationary housing, thereby preventing damage to the shroud.
Although the shroud improves rotor assembly efficiency, the additional mass of conventional shrouds introduces significant stresses during operation, particularly at the root portion of the blade. These stresses can result in high cycle fatigue and potential failure of the rotor assembly. As the component most remote from the axis of rotation, the shroud experiences the maximum angular velocity during operation. This velocity, combined with the additional mass of the conventional shroud, significantly increases the centrifugal force on the rotor blade during operation. The shroud itself experiences significant bending stresses during operation, particularly at the fillet 40, shown in FIG. 2, defined by the intersection of the shroud with the airfoil portion of the blade. Conventional shroud designs compensate for the higher bending stress at the fillet by increasing the thickness T of the shroud at the intersection with the airfoil portion of the blade, as shown in FIG. 2. Because the bending stress decreases farther from the fillet, conventional shroud thickness narrows as the shroud extends circumferentially away from the airfoil portion and is generally a minimum near the shroud circumferential sides. Conventional shroud thicknesses vary based upon design and operating conditions such as the material used to manufacture the blade, the operating temperature, the angular velocity and the distance the shroud overhangs the airfoil in the circumferential direction, but are typically about 0.22 inches at the fillet and 0.12 inches at the circumferential sides for shrouds made of A-286, an iron-based superalloy, and designed to operate at shroud speeds of approximately 1900 feet per second. The additional shroud thickness near the fillet reduces the centrifugal bending stress acting upon the shroud during operation but, due to the additional mass, increases the centrifugal stress on the blade.
Conventional shrouds may optionally provide frictional damping to reduce the magnitude of vibratory stresses induced on rotor assembly blades during operation by limiting the relative motion of adjacent components through frictional rubbing contact. Many techniques are known to achieve the necessary frictional rubbing contact such as direct shroud-to-shroud contact, for longer blades, or the use of frictional dampers, for shorter blades. Several frictional damper designs are known in the art such as blade-to-ground dampers and blade-to-blade dampers. Blade-to-blade dampers can be located anywhere between adjacent blades but are most effective when located radially distal from the axis of rotation because there is greater relative motion between adjacent blades. When located on the conventional shrouds, blade-to-blade dampers are held in place by several known means such as rivets. Frictional dampers are constructed of any suitable damping material, such as metal and may be a single piece or several pieces.
Prior art designs have attempted various methods to reduce centrifugal stresses on rotor blades. U.S. Pat. No. 6,491,498 to Seleski et al., incorporated herein by reference, discloses a turbine blade pocket shroud wherein pockets or voids in the shroud reduce the shroud mass, thereby reducing the magnitude of centrifugal stress on the rotor blade. But additional reductions are desired. U.S. Pat. No. 6,241,471 to Herron, also incorporated herein by reference, discloses a shroud with a reinforcing bar to help prevent high cycle fatigue failures. This design reduces the centrifugal bending stress on the shroud but the additional mass of the reinforcing bars actually increases the magnitude of the centrifugal stress on the blade. Therefore, further improvements to reduce rotor blade centrifugal stresses are required.
A need exists for a rotor blade shroud design with reduced mass to limit centrifugal stresses on the blade during rotor operation and with increased strength to withstand the centrifugal bending stress on the shroud resulting from rotor operation.